How many times do we hear about chain store advice like Petco. Here in the UK we have similar stores. So sorry to hear about your situation.
Firstly you need to get that filter cycled before you even consider adding more fish. Any ammonia reading tell you not to increase the stocking levels. Only when ammonia is zero and nitrite is zero and the nitrate low due to water changes should you move forward. You will need patience whist the tank properly cycles - it could be weeks, but it will happen
Then I would check Ph, Kh
to see what type of fish are best suited to your local conditions. It would be my view that live bearers and tetras have different requirements and don't belong in the same tank. Others might disagree.
I don't think you are overstocked, the old rule of 1" of narrow bodied fish (excluding tails) per gallon would be max stocking. But this does rely on very good maintenance routines. However, I would not add any further fish at this stage adding further to the bio-load.
There are some good threads on 'fish-in tank cycling'. I like to borrow some 'dirty' filter media from a friends tank that is clear of problems. This dirty media (please don't clean it) can be put in your filter or hung on a net in the water flow. The good bacteria will populate your filter that much better.
If you don't already have a test kit I advise that you buy one. The API master test kit and the Kh
additional tests will tell you exactly the health of your tank and lead you towards the purchase of further fish. A personal thing but I do not think live bearers are suited to be with tetras.
Good luck with the cycling. Keep us informed of your progress. We have other members on the forum in a similar position and all because the chain fish shops want to make profit from your failure - nice!